The present invention relates to digital cameras, and in particular to correcting vignetting effects.
The vignetting effect is when the brightness around the edges of a screen is less than the brightness in the center of the screen, creating a halo effect. Professional photographers will sometimes try to intentionally create this effect with an appropriate lens or screen, to produce a softened image at the edges of a picture. However, when this effect appears unintentionally in digital photographs, it is annoying. A variety of techniques have been developed to measure and address vignetting effects.
U.S. Pat. No. 4,884,140 shows an analog circuit for providing vignetting compensation for a video camera using a zoom lens which causes vignetting.
U.S. Pat. No. 5,434,902 shows measuring the vignetting effect for an x-ray examination apparatus by using a constant brightness image. A correction factor is then stored in the memory for each pixel.
U.S. Pat. No. 5,576,797 shows the detection of vignetting effects in a camera with a focus detecting device.
U.S. Pat. No. 5,381,174 shows the correcting of vignetting due to operation of a zoom lens by using a field frequency sawtooth signal.
U.S. Pat. No. 4,816,663 shows the detection of vignetting between a photo taking lens and focus detecting optical system.
Digital cameras for use with a personal computer for personal teleconferencing have become cheaper and cheaper. This puts pressure on the camera manufacturers to use cheaper lenses, which have more of a vignetting effect. In addition, the price pressure forces manufacturers to use fewer semiconductor chips, thus making the use of semiconductor memory to store vignetting corrections undesirable. Accordingly, there is a need for a digital camera which is inexpensive to manufacture and also corrects for vignetting of an inexpensive lens.
The present invention provides a method and apparatus for constructing an inexpensive camera with vignetting correction in the camera electronic hardware. The hardware is programmable through the use of a register storing a constant value corresponding to the amount vignetting of a particular lens used in the camera. The register value is provided to a hardware correction circuit which implements an algorithm which corrects for the amount of vignetting of pixels depending upon the pixel position.
The invention thus allows different lens suppliers or lens types to be used in cameras as they are manufactured, by simply programming a different computed value into the register. The correction is applied on the raw sensor data after it is converted into digital form, near the sensor itself. Thus, the data is corrected prior to the optional collection of statistics on the sensor data and the compression of the data for transmission to a host computer over a bus. The vignetting correction of the invention maintains the resolution of the image and allows real-time performance. It also makes more accurate statistics collection, thus making the automatic gain control (AGC) in the host, and the automatic white balance (AWB) in the host more accurate.
For a further understanding of the nature and advantages of the invention, reference should be made to the following description taken in conjunction with the accompanying drawings.